燕辽地区长城系串岭沟组及团山子组沉积时代的新制约

New constraints on ages of the Chuanlinggou and Tuanshanzi formations of the Changcheng System in the Yan-Liao area in the northern North China Craton

华北克拉通北部燕辽地区是长城系地层出露最全的标准建组地区之一。近年来研究结果表明长城系地层底界年龄比以往所认为的18亿年明显年轻。新的锆石LA-ICP-MSU-Pb测年结果显示,平谷熊耳寨团山子组上部钾质火山岩的锆石207Pb/206Pb加权平均年龄为1637±15Ma(95%置信度,N=15,MSWD=0.75),一致年龄为1641±4Ma(1σ,N=14,MSWD=0.12),限定了团山子组上部的沉积时代。蓟县青山岭侵位于串岭沟组地层内闪长玢岩脉的锆石207Pb/206Pb加权平均年龄为1634±9Ma(95%置信度,N=17,MSWD=0.40),表明串岭沟组地层沉积发生在1634±9Ma之前。结合前人大红峪组火山岩的研究结果还表明,燕辽拗拉槽(裂谷)钾质火山岩的喷发开始于1637±15Ma,在大红峪期(～1625Ma)达到高潮。

The Yan-Liao aulacogen （ rifting） is the best exposed area of the Mesoproterozoic strata in the northern North China Craton （ NCC） and the standard stratigraphic section of the Changcheng System. Recent results show that the lower boundary of the Changcheng System is much younger than 1. 8Ga as previously regarded. Our new zircon LA-ICP-MS U-Pb dating on a potassium-rich volcanic rock in the upper part of the Tuanshanzi Formation in Pinggu area yields a weighted mean 207 Pb / 206 Pb age of 1637 ± 15Ma （ 95% confidence,N =15,MSWD =0. 75） and a concordia age of 1641 ±4Ma （ 1σ,N =14,MSWD =0. 12） ,which represent the sedimentary age of the upper part of the Tuanshanzi Formation. Zircons from a dioritic porphyrite dyke emplaced into the Chuanlinggou Formation in Jixian area yield a weighted mean 207 Pb / 206 Pb age of 1634 ± 9Ma （ 95% confidence,N = 17,MSWD = 0. 40） ,indicating sedimentation of the Chuanlinggou formation prior to 1634 ± 9Ma. Our new results also indicate that eruption of the potassium-rich volcanic rocks in the Yan-Liao aulacogen （ rifting） initiated from 1637 ± 15Ma,and peaked at ca. 1625Ma during the Dahongyu Period as suggested by previous studies in the Dahongyu volcanic rocks.